About filtering
Filtering technology is an effective measure to suppress interference , especially in dealing with the switching power supply signal transduction EMI interference and radiated interference in some respects , has a significant effect.
Any power line conducted interference signals are available differential and common mode interference signal to represent.
Differential mode interference between the two transmission wires belonging to the symmetry of the interference ; common mode interference transmission conductors and ground ( chassis ) between , a non- symmetric interference. In general, the differential mode interference amplitude , low frequency , the smaller the interference caused by the common-mode interference amplitude , high frequency radiation can also be produced by a wire , caused significant interference . Therefore, to weaken the conduction disturbances , the EMI signal control at the relevant EMC standards prescribed limit level or less. Except outside sources of interference suppression , the most effective way is to install EMI filter source input and output switching circuits . General equipment operating frequency of about 10 ~ 50kHz. Many EMC standards for conducted interference level limits are counted from 10kHz . To produce high- frequency switching power supply EMI signals , just select the appropriate decoupling circuit or network structure is relatively simple EMI filters, it is not difficult to meet the filtering effect of compliance with EMC standards.
1.1 transient interference
Refers to the AC line voltage surge occurs , ringing voltage , such as instant spark discharge interference signal , which is characterized by extremely short duration of action , but the magnitude of the high voltage , high energy transients . When source breakdown voltage V (BR) DS - When the transient voltage superimposed on the DC input voltage rectifier filter VI, VI exceeds the internal power switch so that the drain pipe ; transient interference can cause fluctuations in chip switching power supply output voltage , also damage TOPSwitch chip suppression measures must be adopted . Typically , electrostatic discharge (ESD) and electrical fast transient burst (EFT) for digital circuits jeopardized its impact on analog circuits. Electrostatic discharge in 5 - 200MHz frequency range of a strong RF radiation. This radiation energy peaks often occur between the self-excited oscillation 35MHz-45MHz. Many I / O cables are usually resonant frequency in this frequency range as a result, the cable will be the string into a lot of radiation energy electrostatic discharges . When the cable is exposed to 4-8kV ESD environment , I / O cables to the terminal load induced voltage can be measured up to 600V. This voltage is far beyond the typical digital value of the threshold voltage of 0.4V. The typical pulse duration is about sensor 400 ns . The I / O cable shielding and grounding it at both ends , so that all internal signals in the lead shield , interference can be reduced 60-70dB, the induced voltage across the load is only 0.3V or lower. Electrical fast transient burst also have a very strong radiation emission , which is coupled to the cable and chassis lines. Power line filters can be protected against power . Line - common mode capacitance to ground is valid devices such transient interference suppression , which makes the interference bypass to the chassis , and away from the internal circuits. When this capacitor leakage current capacity is restricted but not too large, common mode choke must provide greater protection. This usually requires the use of a dedicated center-tapped common mode choke , center tap connected to the chassis through a capacitor ( capacity is determined by the leakage current ) . Common mode choke is usually around the high permeability ferrite core , the typical inductance value 15 ~ 20mH.
1.2 conduction suppression
Often simply can not provide a complete shielded electromagnetic interference protection , because the cable on the device or system is the most effective interference receiving and transmitting antennas. Many devices have not done a single experiment electromagnetic compatibility problems , but after the two devices are connected , do not meet the requirements for electromagnetic compatibility , and this is the cable played a role in receiving and radiating antenna. The only measure is to add filters, electromagnetic interference along the signal path cut line or power line transmission, and common enough to improve the shielding electromagnetic interference protection , whether it is the source of interference suppression , the ability to eliminate the coupling or increase anti- receiving circuit , can be using filtering techniques. For different interference suppression should adopt different technologies, from simple lines to clean , to individual elements suppressors , filters and transformers, and then to more complex regulator and clean power , as well as expensive and perfect performance uninterruptible power supply, respectively, for the following brief description .
1.3 Private Line
As long as the supply line through a simple clean-up can get some interference suppression. As identified in a three-phase power line interference with sensitive equipment as power supply ; to another phase as the power supply external devices ; then one phase power supply as commonly used test equipment or other auxiliary equipment. Such treatment can avoid mutual interference between devices , but also conducive to three-phase equilibrium . It is worth mentioning that in the modern electronic equipment system, due to the use of non-linear load distribution lines , resulting in the presence of harmonic currents in the line , and zero sequence harmonic component can not cancel each other out in the middle , but is superimposed , so too thin will cause an increase in the middle of the line impedance , interference will also increase . At the same time it will result in too small midline midline overheating.
1.4 Transient Suppressor
There is a transient suppressors gas discharge tubes, metal oxide varistors, silicon diodes and solid discharge transient absorption other . Wherein the metal oxide varistors and silicon diode transient absorption work a bit like an ordinary regulator, is clamped interference absorption device ; while the gas discharge tube and solid discharge is absorbed energy transfer interference devices ( with gas the discharge tube , for example, when there is more than the voltage across the discharge firing voltage of the discharge tube , the gas in the tube ionizes the arc between the electrodes due to low pressure drop of the arc , to make the most of the energy transfer transient to protect the equipment from transient voltage damage ) . Transient suppressors used in parallel with the protected equipment .
1.5 Gas Discharge Tubes
Gas discharge tubes , also known as lightning tubes, currently used on the PBX . Lightning surge absorber tube has a strong ability of high insulation resistance and the parasitic capacitance is small , the normal work of the apparatus will not have any harmful effects. But its response to the surge arcing , and arcing between the DC voltage in response to the presence of significant differences. For example 90V DC gas discharge tube arcing voltage is 90V, while the 5kV/μs the maximum surge voltage arcing may reach 1000V. This indicates that the response speed of the gas tube surge voltage is low. Therefore, it is more suitable as a protection lines and equipment . Furthermore, the gas discharge voltage grade small.
1.6 Metal oxide varistors
Because of cheap , varistors are now widely used transient absorption interference devices . Describe the main parameters of the varistor performance is the nominal voltage and current capacity varistors that surge current absorption capacity. The former is an argument often easy to confuse the user . The nominal voltage of the varistor under the condition of constant current means ( 7mm below the outer diameter of the varistor taken 0.1mA; 7mm taken over 1mA) in the voltage drop across the varistor . Varistors have a greater dynamic resistance , impact of the predetermined shape of the current ( usually 8/20μs impulse current standard ) in the voltage across the varistor ( also known as the maximum voltage limit ) is approximately varistor 1.8 to 2 times the nominal voltage ( this value is also known as residual pressure ratio ) . This requires that the user has previously estimated in the choice of varistors, for indeed it is possible to have greater impact on the current situation , you should choose to use dimensions larger device ( varistor current is proportional to the absorption capacity of the device flow area is proportional to the thickness of the device withstand voltage , and the absorbed energy is proportional to the volume of the device ) . It should be noted the use of varistors inherent capacitance. Depending on the dimensions and nominal voltage , capacitance between several thousand to several hundred pF, which means varistor inappropriate use in high frequency applications , more suitable for frequency applications, such as thyristors and power into the line for protection . In particular, note that the high-speed performance ( up to ns) level , it must be noted that it is installed varistor inductive effect of lead , long lead varistors for transient absorption interference will be introduced when the lead inductance generated due to induction voltage ( on the oscilloscope , the induced voltage spikes were like ) . The longer lead , the greater the induced voltage . In order to obtain a satisfactory interference suppression effect , should try to shorten their lead . About varistor voltage selection , might want to consider some of the protected line voltage fluctuations ( generally take 1.2 to 1.4 times ) . If the AC circuit , but also pay attention to the relationship between voltage rms and peak . So for 220V line , the selected nominal varistor voltage should be 220 × 1.4 × 1.4 ≈ 430V. In addition, the current absorption capacity varistor for , 1kA ( for 8/20μs current wave ) used in the thyristor protection , 3kA surge absorber used in electrical equipment ; 5kA and electronic equipment used in the lightning overvoltage absorption ; 10kA used in lightning protection. Varistor voltage grades are more suitable for primary or secondary protection device .
1.7 silicon transient voltage absorbing diode (TVS tube )
Transient voltage silicon diode with fast absorption response time ( nanosecond ) and a very high surge absorption capacity , and very much voltage grade. Equipment or circuits can be used to protect against static electricity , voltage transients generated when an inductive load switching and lightning overvoltages generated .
TVS has a single direction ( single diode ) and ( two diodes connected back to back ) to the two kinds of parties , their main argument is the breakdown voltage, leakage current and capacitance. Use than the breakdown voltage TVS protection circuit tube is about 10% higher operating voltage to prevent voltage close to the line of work TVS breakdown voltage, so that the leakage current TVS affect the circuit to work ; also avoided due to environmental temperature changes cause TVS tube breakdown voltage fall into line normal operating voltage range.
TVS tube has a variety of packages , such as axial lead product is available on the power feeder ; dual in-line and is suitable for surface mount printed circuit board as a protection logic , I / O bus and the data bus.
TVS tube matters in use should be noted :
· On transient voltage between the absorbed power ( peak ) and transient voltage pulse width relationship. Manual to just absorbed power ( peak ) under specific pulse width , and pulse width of the actual lines are unpredictable , should be estimated in advance . On the pulse width should be derated .
· To protect small current load can consciously increase the current limiting resistor in the circuit , as long as the appropriate current limiting resistor , the line will not affect the normal operation , but the current limiting resistor interference generated but it will greatly reduce the small . This may choose a smaller peak power TVS tubes for small current load circuit protection.
· To repeated transient voltage suppression , particularly noteworthy is the steady-state average power TVS tube is within the safe range .
· TVS tube as a semiconductor device , pay attention to environmental temperature derating when using problem. · Particular attention to the TVS tube length lead , and it is the relative distance of the protected line .
• When no suitable voltage TVS tube for adoption , allowing multiple TVS tube series. The maximum current series tube depends on the tube current sink capability using the smallest one . The absorption peak of this current series with power equal to the sum of the product of tube voltage .
· TVS tube junction capacitance is a key factor affecting its use in high-speed line , and in this case , generally with a tube and a fast recovery TVS diode connected back to back , the fast recovery diode junction capacitance smaller thus both series equivalent capacitance is small , you can meet the requirements of high-frequency use.
· Solid discharge
Solid discharge is a relatively new transient absorption interference device with fast response (10 ~ 20ns level ) , the absorption current is large , the operating voltage stability and long service life. Solid discharge and gas discharge tubes belong to energy transfer . Figure 2.2 for VA properties. When outside interference below the trigger voltage , the tube was cut shape. Once the interference exceeds the trigger voltage , the voltage characteristic occurs turning into negative resistance area where the current is great, and the on-resistance is extremely small, so that the energy can be transferred interference . With the interference is reduced by lower discharge current, discharge time while maintaining the current, low-impedance discharge quickly out of the area by currents below , and return to a high impedance state , to complete a discharge process . One advantage is its solid discharge short circuit failure mode ( when the device failure , short circuit between two electrodes was like ) , is necessary for many applications , has been widely used at home and abroad . Solid discharge voltage grade fewer , more suitable for protecting networks, communications equipment, and even parts level.
Seven , PCB tips
1, the components automatically generated label or labeling of existing components cancel again
Tools Tools | Annotate ... Comment
All Part: generate labels for all components
Reset Designators: removal of all components label
2, single- panel settings :
Design Design | Rules ... Rules | Routing layers
Toplayer set NotUsed
Make Any Bottomlayer
3 , automatic routing the power cable before setting bold
Design Design | Rules ... Rules | Width Constraint
Increase : NET, select the network name VCC GND, rough set width
4, PCB packaging updates, as long as the original package footprint right inside the window to pop up a new package No.
5,100 mil = 2.54mm; 1mil = 1/1000英寸
6 , the shortcut "M", Dram Track End drop-down menu in the end point of dragging the endpoints ==== pull PCB interconnects continue the connection.
7 , the positioning holes
In KeepOutLayer layer ( prohibition wiring layer ) in a circle , Place | Arc ( circle arc ) center, radius and then adjust its position
8 , set the drawing parameters
Design | Options | Sheet Options
( 1 ) Set Paper Size : Standard Sytle choice
( 2 ) Set the drawing direction : Orientation option ---- Landscape ( squares direction ) ---- Portrait ( vertical direction )
( 3 ) Set the drawing title bar (Title BlocK): Choose Standard for the standard , ANSI is the American National Standard Institute
( 4 ) Set the display reference frame Show Reference Zones
( 5 ) Set the display drawing border Show Border
( 6 ) Set the display drawing template graphics Show Template Graphics
( 7 ) Set the drawing grid Grids
Lock Raster Snap On, visual grid set Visible
( 8 ) is set to automatically find electrical node
10 , the component of rotation :
Space key: the selected element rotated 90
In reversing the PCB devices (such as digital ) , select the original forward device , drag or selected,
X key: the element around swap ( horizontal ); Y key: the element down swap ( vertical )
11 , component attributes:
Lib Ref: Component library model , does not allow members to modify
Footprint: package components
Designator: element number as U1
Part type: component models ( such as the name of AT89C52 chip resistor 10K or so ) ( in the schematic is the case, this change in the PCB to Comment)
12 , to generate a list of elements ( ie, components list ) Reports | Bill of Material
13 , schematic electrical rule test (Electrical Rules Check) that ERC
Is the use of circuit design software designed for users to test the circuit to be able to check out the human error or negligence .
Schematic drawing window Tools Tools | ERC ... Electrical Rule Check
ERC dialog box to define the options :
Multiple net names on net: Detection " on the same network name more than one network name " error
Unconnected net labels: " label is not actually connected to the network " warning checks
Unconnected power objects: " is not actually connected to the power maps " warning checks
Duplicate sheet mnmbets: Detect " circuit diagram numbering No. "
Duplicate component designator: " element numbering No. "
bus label format errors: " Bus label format error "
Floating input pins: " input pin is floating ."
Suppress warnings: " Detections will ignore all warnings of detections , with a test report does not display a warning wrong."
Create report file: " After the test program is executed automatically the presence of the test results report file "
Add error markers: whether the error symbols are automatically placed in the wrong location
Descend into sheet parts: the test results broken down to each schematic for the purposes of hierarchical schematics
Select the desired range for testing schematic file : Sheets to Netlist
Net Identifier Scope: select the network identifier range
14 , the system of the original library Miscellanous Devices.ddb in DIODE ( two tubes ) package should be changed , but also put the pin description
1 (A) 2 (K) to A (A) K (K)
PCB netlist import this painting was not a mistake : Note Not Found
15 , PCB layout follows the principles
( 1 ) the input and output wire should avoid using adjacent parallel . Plus the best line between the ground to avoid feedback coupling .
( 2 ) the minimum width of the printed wire taken mainly by the adhesive strength between the conductor and the insulating substrate and the trigger current value flowing through their decisions.
When the foil thickness of 0.05mm, while the width is 1 ~ 15mm . 2A current through the temperature no higher than 3 ℃, therefore wire width of 1.5mm (60mil) to meet the requirements. For integrated circuits, digital circuits in particular , usually on 0.02 ~ 0.3mm (0.8 ~ 12mil) wire width . Of course , as long as allowed , or as much as possible with a wide line. Especially the power and ground . The minimum spacing between conductors is mainly determined by the worst-case line insulation resistance and breakdown voltage. For integrated circuits, digital circuits , especially as long as the process allows , to make small spacing 5 ~ 8mm.
( 3 ) printed conductors and generally arc-shaped bend , and right-angle or angle in high-frequency circuits can affect electrical performance. Also, try to avoid using a large area of copper foil , otherwise . A long time to heat , easy expansion and foil off phenomenon. When a copper foil with a large area must , preferably grid-like , so that there is between the copper foil and the substrate is conducive to exclude volatile gas generated by heating the adhesive .
( 4 ) pad : pad center hole diameter slightly larger than the device leads . Pad is too easy to form Weld. Usually the pad is not less than the outer diameter D (d +1.2) mm, where d is the lead diameter . High density digital circuits , preferably the smallest diameter pads (d +1.0) mm.
16 , the working level Type Description
⑴, signal layers (Signal Layers), there are 16 signal layers , TopLayer BottomLayer MidLayer1-14.
⑵, internal power supply / ground layer (Internal Planes), there are four power / ground layer Planel1-4.
⑶, mechanical layer (Mechanical Layers), there are four mechanical layer .
⑷, drilling position layer (Drill Layers), mainly used for mapping and borehole drilling location map, including a total of Drill Guide and Drill drawing layers.
⑸, flux layer (Solder Mask), there TopSolderMask and BottomSolderMask floors, hand- tin.
⑹, Paste protective layer (Paste Mask) has TopPaste and BottomPaster layers.
⑺, silkscreen layer (Silkscreen), there TopOverLayer and BottomOverLayer two main components used to draw the outline .
⑻, other working level (Other):
KeepOutLayer: ban wiring layer to the outer boundary of the hollow part of the draw and the positioning hole printed circuit board , etc.
MultiLayer: Multilayer
Connect: Session Layer
DRCError: DRC error layer
VisibleGrid: visual raster layer
Pad Holes: pad layer
Via Holes: vias layer
